The prior art in this field teaches dental apparatus which remove fluids from a patient's mouth during an operation. These fluids comprise a gaseous part and a liquid part. The gaseous part is generally air, while the liquid part is usually composed of water, organic liquids and other liquids used in dental apparatus. The gaseous part has to be separated from the liquid part before the latter is purified and ultimately discharged into the sewers. The prior art teaches separators for this purpose, which combine the action of a centrifuge drainage pump with that of a suction pump. In these separators the fluids which are to be separated are sent into the centrifuge pump; the suction pump creates a depression inside the centrifuge pump causing the gaseous part of the fluid to be aspirated by the suction pump, while the liquid part exits through an aperture afforded in the centrifuge pump. The prior art teaches separators, for example in European Publications EP 0 237 708 and EP 0 766 008, in which a single motor is used to drive both the centrifugal and the suction pumps.
In these known separators, however, the suction pump might aspirate some of the liquid, especially when the separator is started up. This happens mainly because at the moment when the separator is stopped, the liquid remaining in the tubes of the separator itself might flow towards the drainage pump and collect in the bottom thereof, eventually reaching a certain level so that when the separator is started up again a part of the liquid can be sucked up by the suction pump. This both damages the suction pump and may cause dispersion of tiny droplets of a pollutant which is expelled from the suction pump and thus constitutes an environmental risk.